1. Field of the Invention
The present invention relates to switching regulators capable of increasing the efficiency under light load.
2. Description of the Related Art
Switching regulators are used as a voltage supply source for various electronic device circuits. One of functions of switching regulators is to output a constant voltage through an output terminal with high conversion efficiency irrespective of voltage fluctuation at an input terminal. It is also important for switching regulators to maintain high conversion efficiency even when an electronic device as a load enters a low power consumption mode such as a stand-by state so that current supplied from the output terminal to the load decreases.
FIG. 4 is a circuit diagram of a conventional step-up switching regulator.
An input power supply 20 is connected to a coil 22. A rectifying device 23 is connected between the coil 22 and an output capacity 24. A load 25 is connected in parallel with the output capacity 24. A switching regulator control circuit 200 controls ON and OFF of a switching element 21 of a switching regulator.
The output from an error amplifier 13 is voltage Verr, the output from a reference voltage circuit 10 is reference voltage Vref, voltage at a connection between bleeders 11 and 12 is divided voltage Vfb. When Vref>Vfb, the voltage Verr becomes high, and conversely when Vref<Vfb, the voltage Verr becomes low. A PWM comparator 14 compares the output Vramp (e.g., triangle wave) from an oscillation circuit 15 with the voltage Verr and outputs a signal. FIG. 5 illustrates a relationship among these signals. That is, increase/decrease of the voltage Verr output from the error amplifier 13 controls a pulse width of an output signal Vpwm of the PWM comparator 14. This is so-called PWM control by a switching regulator.
Generally longer ON duration of a switching element allows a switching regulator to feed much power to a load. For instance, larger load current Iout decreases the output voltage from the switching regulator, thus decreasing the divided voltage Vfb. This causes the voltage Verr to increase, thus widening the output pulse width of the PWM comparator 14, and the pulse width is controlled so that the output voltage Vout can be kept constant. Conversely, smaller load current Tout increases the output voltage from the switching regulator, thus increasing the divide voltage Vfb. This causes the voltage Verr to decrease, thus narrowing the output pulse width of the PWM comparator 14, and the pulse width is controlled so that the output voltage Vout can be kept constant. In this way, the voltage Verr output from the error amplifier 13 changes with the load current value, thus controlling the pulse width of the switching regulator.
The aforementioned PWM control, however, has a drawback that smaller load current Tout (hereinafter called light load) degrades the efficiency extremely. This is because the amount of energy necessary to turn ON/OFF of a switching element increases with reference to the supply energy to the output. It is widely known that average current I flowing during repeated charge and discharge at a frequency of f [Hz] from 0 [V] to V [V] satisfies I=CVf [A]. That is, let that the input capacity of the switching element 21 is Cin [F], the voltage of the input power supply 20 is Vin [V] and the switching regulator operates at the frequency of Fosc [Hz], a current value Top necessary to drive the switching element 21 will be Iop=Cin×Vin×Fosc.
As one example, in the case of typical numerical values such as Cin=500 pF, Vin=5 V and Fosc=1 MHz, Iop will be 2.5 mA and power loss Pin on the input power supply side will be Pin=Vin×Iop=12.5 mW. At this time, when the output voltage Vout is 10 V and the load current Tout is 1 mA, the supply power Pout to the output side will be Pout=Iout×Vout=10 mW. Accordingly, just power necessary to drive the switching element 21 exceeds the supply power to the output. Actually flow-through current of a buffer circuit and the like also flows to drive the switching element, and therefore loss involved in the driving of the switching element becomes a much larger value.
Such losses involved in the driving of the switching element, however, have a property of increasing with an operating frequency of the switching regulator, and therefore conventionally a technique to lower the operating frequency thereof under light load has been widely used, thus reducing a driving loss of the switching element 21. As mentioned before, since the voltage Verr varies with the load current, thereby controlling the output voltage Vout, output load detection means can be configured by monitoring the voltage Verr. Based on this, a load detection circuit 100 of FIG. 4 is configured. The load detection circuit 100 is made up of transistors 110 and 111. As the voltage Verr of the error amplifier 13 increases, gate voltage of the Nch transistor 110 increases, and therefore current Iosc1 between drain and source thereof increases. Since the transistor 111 and a transistor 112 have a current relationship, current Iosc2 between drain and source of the transistor 112 also increases in proportion to the current Iosc1. The current Iosc2 is bias current of the oscillation circuit 15, and when the oscillation circuit 15 is configured so as to oscillate using time to charge the capacity with the current Iosc2, for example, the oscillation frequency changes depending on the current Iosc2.
For example, when the load current Tout becomes large and the output voltage Vout decreases, the voltage Verr increases. As a result, the current Iosc1 and the current Iosc2 increase. Therefore the oscillation circuit 15 oscillates at a high frequency. Conversely in the case of light load, the voltage Verr decreases and the current Iosc2 reduces, and therefore the oscillation frequency decreases, so that the frequency of ON/OFF of the switching element reduces. As a result, driving loss reduces, whereby efficiency under light load can be improved (see Patent Document 1, for example).    [Patent Document 1] Japanese Patent Application Laid-Open No. 11 (1999)-155281